1. Technical Field
The present invention relates to a stator blade for a gas turbine, which is enabled by improving the feed of sealing air to reduce leakage of the air thereby to feed the air to an inner shroud efficiently and by cooling the sealing air to reduce the clearance between a rotor side and a stationary side at a rated running time.
2. Description of Related Art
FIG. 14 is a general block diagram of a gas turbine, which is constructed to include a compressor 150, a turbine 151 and a combustor 152. A fuel is burned in the combustor 152 with the air coming from the compressor 150 so that a hot combustion gas is fed to the turbine 151. This combustion gas flows through a combustion gas passage, in which moving blades mounted on a rotor and stator blades are alternately arranged in multiple stages, to rotate the rotor thereby to drive a power generator connected directly to the rotor. Since the turbine 151 is exposed to the hot combustion gas, the air from the compressor 150 is partially bled and fed to the turbine 151 to cool the stator blades, the moving blades and the rotor.
FIG. 15 is a sectional view showing a sealing air feed line to a representative stator blade of a prior art gas turbine, and shows the construction of the blades in the turbine 151 of FIG. 14.
In FIG. 15, reference numeral 21 designates a moving blade including a platform 22, a seal plate 23 under the platform 22, two end portions 24 and 25 of the platform 22, and a blade root 26. A plurality of moving blades 21 each composed of those members, are arranged in the circumferential direction of the rotor.
Reference numeral 31 designates a stator blade which is arranged adjacent to the moving blade 21. Numeral 32 designates an outer shroud, and numeral 33 designates an inner shroud. Numerals 34 and 35 designate two end portions of the inner shroud 33, and numeral 36 designates a cavity under the inner shroud 33. Numeral 37 designates a seal ring retaining, ring which has a labyrinth seal 37a at its end portion and which slides with respect to the rotating portion of the blade root 26 on the moving blade side. Numeral 38 designates an air hole that is formed through the seal ring retaining ring 37 to provide communication between the cavity 36 and a space at the blade root 26 of the adjoining moving blade 21. Numerals 40a and 40b designate seal portions between the platform 22 and the inner shroud 33. The seal portions adjoining each other and are constructed by fitting seal members between the end portions 24 and 34, and the end portions 25 and 35.
Numeral 50 designates a blade ring, on the inner side of which the outer shroud 32 of the stator blade 31 is fixed through heat insulating rings 32a and 32b. Numeral 51 designates an air hole, which is formed in the blade ring 50. The air hole 51 communicates with a space 53, which is formed by the blade ring 50, the heat insulating rings 32a and 32b and the outer shroud 32. The space 53 is connected at its leading end with an air source leading from the not-shown compressor. Numeral 52 designates a seal tube which extends from the outer shroud 32 in the stator blade 31 through the inner shroud 33.
In the construction thus far described, cooling air 54 from the compressor is fed from the air hole 51 of the blade ring 50 and into a space 53. This cooling air 54 flows on one side through the seal tube 52 into the cavity 36 under the inner shroud 33. The cooling air from this cavity is blown from the air hole 38, as indicated by arrow S1, into the trailing side space of the adjoining moving blade 21 at the upstream side and further through the labyrinth seal 37a into the leading side space of the moving blade 21 at the trailing stage, as indicated by arrow S2. These cooling air flows S1 and S2 emanate from the seal portions 40a and 40b, respectively, to prevent the combustion gas from entering the inside of the inner shroud 33.
As shown in FIG. 16, the air that has entered the space 53 cools the face of the outer shroud 32 and enters the cooling passage in the stator blade, so that it is blown out of the holes of the trailing edge while cooling the blade inside, until it is released into the combustion gas passage.
In the sealing structure thus far described, the air hole 51 of the blade ring has a diameter of 2 to 50 mm, and the seal tube 52 is limited in its internal diameter by the thickness and the camber of the blades. As a result, the in flow of air is subjected to a pressure loss so that its pressure drops. In addition, the cooling air having entered the space 53 leaks from clearances between the outer shroud 32 and the heat insulating rings 32a and 32b, as indicated by arrows S3 and S4.
One example of the pressure situations resulting from the aforementioned leakage will now be described. The cooling air 54 flowing into the air hole 51 of the blade ring 50 has a pressure of about 6 Kg /cm.sup.2. This pressure is lowered to about 5 Kg/cm.sup.2 in the space 53 by the pressure loss and further to 3.5 Kg/cm.sup.2 in the cavity 36 due to the pressure loss. This pressure level is equal to the pressure of 3.5 Kg/cm.sup.2 between between the moving blade 21 and the stator blade 31 adjoining each other so that the sealing effect is deteriorated.
A first problem of the sealing structure for the prior art gas turbine stator blade thus far described, is that the cooling air fed from the air hole 51 of the blade ring 50 leaks from the clearances between the outer shroud 32 and the heat insulating rings 32a and 32b, even though it flows into the space 53 between the blade ring 50 and the outer shroud 32 and into the cavity 36 under the inner shroud 33 from the seal tube 52. On the other hand, the cooling air is subjected to a pressure loss in the seal tube 52 so that its pressure drops when it flows into the cavity 36 of the inner shroud. As a result, the difference in the pressure of the combustion gas disappears to make it difficult for the cooling air to maintain sufficient pressure as the sealing air.
FIG. 16 is a sectional view showing a stator blade of the prior art gas turbine and explains the cooling of the stator blade mainly although the stator blade has the same structure as that of FIG. 15. In the stator blade 31, as shown in FIG. 16, air passages 80A, 80B and 80C are sequentially formed to form a serpentine passage. Reference numeral 80D designates the trailing edge of the blade, which has a number of film cooling air holes 60. The seal tube 52 vertically extends through the stator blade 31. The seal tube 52 opens at its lower end into cavity 36. The seal ring retaining ring 37 retains the flange of the inner shroud 33 and the labyrinth seal 37a. The air hole 38 is formed in the retaining ring 37 to provide communication between the cavity 36 and a space 72 between the adjoining moving blade. The outer shroud 32 has a cooling air feeding hole 62. Note the numeral 21 designates the adjoining moving blade 21.
In the stator blade thus constructed, cooling air 70 is fed from the hole 62 of the outer shroud 32 to the air passage 80A on the leading edge side of the stator blade 31, and the air then flows at the inner side into the next air passage 80B and then at the outer side into the adjoining air passage 80C. The cooling air 70 then flows at the inner side to cool the stator blade 31 sequentially and eventually flows from the air holes 60 of the trailing edge 80D along the outer surfaces of the trailing edge to provide a film cooling effect.
From the open end of the seal tube 52 of the outer shroud 32, cooling air 71 for the cooling operation flows from the lower end of the seal tube 52 into the cavity 36, as shown in FIG. 16, and this portion of the air flows from the air hole 38 formed in the cavity 36 into the space 72 between the former and the adjoining moving blade and further through the labyrinth seal 37a into a forward space 73. Thus, the cooling air flows from the seal tube 52 into the cavity 36 to keep the inside of the cavity 36 at a higher pressure level than that in the outside combustion gas passage to thereby prevent the hot combustion gas from entering the interior of spaces 72, 73. Although not shown, the air from the compressor passes the disc cavity and flows from the radial hole formed in the blade root 26 to the inside of the platform 22 and is guided to cool the moving blade 21.
The prior art gas turbine stator blade is provided with the air passage for cooling, as has been described hereinbefore. This air passage is ordinarily formed as a serpentine passage, and the cooling air is fed from the outer shroud into the air passage to cool the inside of the stator blade until it is released from the trailing edge. Separately of the cooling purpose, the seal tube extends through the stator blade to feed a portion of the cooling air as the sealing air from the outer shroud into the cavity of the inner shroud, which is kept at a higher pressure than that in the external combustion gas passage to prevent the hot combustion gas from entering the cavity of the inner shroud.
In the stator blade cooling system thus far described, the cooling air is fed for the cooling purpose and for the sealing purpose. The cooling air cools the stator blade and is then released from the trailing edge to the combustion gas passage. On the other hand, a portion of the cooling air is bled as sealing air and is fed through the seal tube to the cavity so that it is released from the cavity into the spaces between the former and the adjoining front and rear moving blades. In addition to the pressure loss of the foregoing first embodiment, therefore, a second problem of the sealing device for the gas turbine stator blade arises from the fact that large amounts of air are consumed for the cooling and sealing purposes so that the capacity of the compressor must be increased thereby adversely affecting the performance of the gas turbine.
FIG. 17 is a section showing a general blade cascade of the gas turbine and the entire cascade of the stator blades shown in FIG. 15 or 16. Reference numerals 81C, 82C, 83C and 84C in FIG. 17 designate the stator blades, which are individually arranged in a plurality of radial rows around the rotor and on the stationary side. Numerals 81S, 82S, 83S and 84S designate the moving blades which are mounted around the rotor through their respective roots and which are axially arranged alternately of the rows of stator blades so that they may rotate together with the rotor. Numerals 111C, 112C, 113C and 114C designate the individual inner shrouds of the stator blades 81C to 84C, respectively, and numerals 1115, 1125, 1135 and 1145 designate the individual platforms of the moving blades 81S to 84S.
Numerals 37-1, 37-2 and 37-3 designate the seal ring retaining rings, which are respectively fixed on the flanges of the inner shrouds 111C to 114C of the stator blades 81C to 84C and are arranged in an annular shape around the rotor. These seal ring retaining rings 37-1 to 37-3 retain on their interiors the labyrinth seals (or seal rings) adjacent to the rotor. Thus, in the example shown in FIG. 17, the gas turbine is constructed of the stator blades and the moving blades of four stages. With this construction, the rotor is rotated by the combustion gas to drive a generator.
In the gas turbine, as described with reference to FIGS. 15 and 16, the stator blades, the moving blades and the rotor are exposed to the hot gas which is 800 to 1,000.degree. C. at the entrance or up to 1,500.degree. C., as developed in recent years, so that these components are cooled by the cooling air by bleeding the air from the compressor. A constant clearance is necessary between the rotor side and the stationary side of the members. In FIG. 17, a clearance CR' is maintained between the lower end of the labyrinth seal 37a, supported by the seal ring retaining ring 37, and the opposing face on the rotor side. Between the individual turbine stages, the clearance CR' is a minimum from the start to the rated rotating speed by the time difference of the thermal elongation between the rotor side and the stationary side and increases from the minimum when the rated rotating speed is reached as the clearance is heated by the combustion gas. The clearance CR' is preferably small for the higher sealing performance. Since the clearance is at a minimum after the start by the aforementioned characteristics, however, the design value cannot be made so small while estimating not only that minimum but also the vibration during the run and manufacture error. Therefore, a third problem is that a large clearance will cause the sealing performance to deteriorate when the rotation reaches the rated value. Thus, there is a need to optimize the clearance in order to improve the drop of sealing pressure due to the pressure loss associated with the aforementioned first problem and the consumption of the high flow rate of the air in the second embodiment and to reduce the clearance during the run.